This work proposes a novel UHF RFID-assisted axle counting system for wireless train detection. In the system, the RFID tag antennas are proposed to be attached to the train compartments. When the train travels across the signal indicator next to the railway, the reader detects the RFID tag, and the axle counter counts the number of passing train axles. To overcome the traditional reader antenna with the blind reading zone that may introduce safety problems, a new null-filled monopole UHF RFID reader antenna is proposed. The monopole is connected to the metal ring, which changes the antenna's surface current and electric field distribution, and eliminates the radiation blind reading zone at the antenna zenith. The measured S11 of the proposed antenna is better than -10 dB in 0.84-1.02 GHz, achieving a relative bandwidth of 19.6%. The antenna has a zenith gain of 1.5 dBi, with horizontal radiation gain better than -3.55 dBi and an HPBW of 141° at 0.92 GHz. The null-filled monopole antenna can provide a potential solution for the reader in the proposed UHF RFID-assisted axle counting system under wireless train detection.